Sensor and method for mesaurement of select components of a material

ABSTRACT

A sensor and method is provided for measuring one or more select components of a material. In one embodiment, a method measures the components by emitting electromagnetic radiation at the material and detecting the intensity of the emerging radiation at separate locations from the source. In another embodiment, a sensor provides a radiation source for emitting radiation at a sheet, a plurality of detecting means, offset substantially the same from the source, for detecting radiation after interaction with the sheet and first and second reflectors for directing the radiation so that the radiation makes multiple interactions with the sheet when moving from the source to the detecting means. The invention can accurately measure the select components (e.g., moisture) of different grades of paper by eliminating the effects of the scattering power and determining absorption power at each band of the spectrum considered necessary for a particular measurement.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to the field of sensors and methods for measuringone or more select components of a material. In particular, theinvention relates to measuring the components by emittingelectromagnetic radiation at the material and detecting the amount ofemerging radiation at separate locations. The invention can accuratelymeasure the components (e.g., moisture) of different grades of paper byeliminating the effects of the scattering power and determiningabsorption power at each band of the spectrum necessary for theparticular measurement.

(2) Description of the Related Art

Because paper is produced in a sheet from an aqueous suspension, whichincludes wood pulp fibers, cotton fibers and various chemicals, itinitially contains a considerable amount of moisture. Most of thismoisture is removed during paper production. However, for a variety ofreasons, it is often desirable to include at least some moisture in thepaper. For example, if the paper is too dry, it will tend to curl at theedges or may increase the cost of production.

A paper sheet is typically dried by passing it around heated dryingdrums. However, this tends to dry the sheet unevenly across itscross-direction width, producing paper of uneven quality. Devices havebeen developed to selectively moisten or dry the cross-directionalsections of the sheet. U.S. Pat. No. 5,020,469 to Boissevain et al.,assigned to Measurex Corporation, describes such a device. Typically,the moistening or drying occurs after the sheet has passed around thedrying drums. Of course, the paper mill operator, or the paper mill'sprocess control computer, must determine the cross-directional moistureprofile of the sheet before these devices can be used effectively. Thus,moisture sensors have been developed to measure the cross-directionalmoisture profile.

Water absorbs electromagnetic radiation across the infrared spectrum asa function of wavelength. Some moisture sensors take advantage of thisphenomenon by emitting infrared radiation at the sheet and detecting theamount of the radiation passing through or reflected from the sheet ator near the water absorption peak. The more moisture in the sheet, theless radiation at or near the water absorption peak that will passthrough or be reflected from the sheet.

An infrared moisture sensor can be set up with an infrared radiationsource located on one side of the sheet and two detectors on theopposite side. Each detector has an associated band pass filterpositioned between the source and the detector so that the detector onlyreceives radiation in a select band of the spectrum. A first band passfilter passes that portion of radiation which is near a water absorptionpeak to a first detector. Thus, the first detector is primarilysensitive to the amount of water in the sheet and receives more infraredradiation when the sheet is dry and less infrared radiation when thesheet is moist.

A second band pass filter passes radiation in a band of the spectrumwhere there is less moisture absorption. In this band, most of theabsorption is from sheet fibers rather than moisture in the sheet. Thus,when the basis weight (i.e., weight per unit area) of the sheet fiberincreases, the second detector receives less infrared radiation. Theoutput of the second detector corrects for changes in the basis weightof the sheet fiber. When the outputs from these two detectors areproperly combined, the sensor provides an accurate measurement of themoisture in the sheet so that the changes in the basis weight of thesheet fiber do not affect the moisture measurement.

U.S. Pat. No. 4,928,013 to Howarth et al., assigned to MeasurexCorporation, describes an infrared moisture sensor of this type with twoband pass filters that are selected to compensate for sheet temperaturechanges which shift the absorption spectrum to either shorter or longerwavelengths. In this sensor, a first band pass filter, associated with ameasure detector, is selected so that it is surrounds the waterabsorption peak at about 1.93 microns. When the sheet temperatureincreases, the intensity of radiation increases at the long wavelengthside of the pass band filter while an approximately equal decreaseoccurs at the short wavelength side. Accordingly, the amount of infraredradiation reaching the measure detector remains substantially constantwhen the sheet temperature changes. A second band pass filter,associated with a reference detector, is selected so that it is in aband of the infrared spectrum that is predominantly absorbed by thesheet fibers. The intensity of the radiation detected by the referencedetector primarily indicates the basis weight of the sheet.

However, the intensity of the detected radiation is not only dependentupon the moisture, basis weight and temperature of the sheet. Each gradeof sheet has its scattering and absorption powers that affect theintensity of the detected radiation. A scattering power of a materialdefines its ability to change the direction of light incident upon thematerial from either the line of incidence when transmitted through orfrom a specular direction when reflected from the material. Anabsorption power defines the material's ability to absorb the incidentlight rather than allow it to be transmitted through or reflected fromthe sheet.

The source of the wood fiber used to make paper products may affect thevalue of the scattering coefficient and/or the broadband absorptioncoefficients. This in turn may affect the accuracy of an infraredmoisture sensor. Changes in the scattering power of paper are oftencaused when the source of the paper pulp changes from one species ofwood to another or from virgin to recycled fiber. Broadband absorptionchange may be caused by the carbon black in printer inks used inrecycled paper or added to colored paper.

U.S. Pat. No. 3,793,524 to Howarth, assigned to Measurex Corporation,describes an infrared moisture sensor for measuring the moisture of asheet of material such as paper. The moisture sensor includes aninfrared source that directs infrared radiation out of an aperturethrough paper and into another aperture to a detector. The source anddetector apertures are located in opposing reflective paper guidesdisposed on either side of the paper and are offset from one another sothat the radiation is reflected repeatedly back and forth between thepaper guides in traveling from the source aperture and to the detectoraperture (FIG. 2). The offset geometry results in relatively lowsensitivity to the scattering power of the paper but may requirecalibration to measure different grades of paper. It would be highlydesirable to have a moisture sensor which has one calibration for abroad range of grades of paper. To achieve this goal any sensitivity tothe scattering power must be eliminated.

SUMMARY OF THE INVENTION

The present invention relates to a sensor and method for measuring oneor more select components (e.g., moisture) of a material by emittingradiation at the material and detecting the amount of radiation emergingfrom the material at separate locations from the radiation source afterthe radiation has multiple interactions with the material.

In one embodiment, the invention provides a sensor for measuring selectcomponents of a material such as a sheet, including: (1) a radiationsource for emitting radiation at a sheet; (2) a plurality of detectingmeans, with substantially the same offset from the source, for detectingradiation after interaction with the sheet; and (3) means for directingthe radiation so that the radiation makes multiple interactions with thesheet when moving from the source to the detecting means.

In another embodiment, the invention provides a sensor for measuringselect components such as the moisture of a sheet, including: (1) asource for emitting radiation through a source aperture; (2) means fordetecting radiation after multiple interactions with the sheet,including at least two apertures, with the substantially the same offsetfrom the source aperture, for receiving the radiation; and (3) first andsecond reflector means for directing the radiation so that the radiationhas multiple interactions with the sheet when moving from the source tothe detecting means.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the invention will become moreapparent from the following detailed description of the preferredembodiments taken in conjunction with accompanying drawings, in which:

FIG. 1 is a partial perspective view of a sensor mounted on a scannerwhich moves back and forth in the cross-direction across the sheet.

FIG. 2 is a schematic elevation view illustrating an embodiment of asensor according to the present invention.

FIG. 3a illustrates a reflective paper guide of the lower head of thesensor with a source and detector aperture.

FIG. 3b illustrates a reflective paper guide of the upper head of thesensor with a detector aperture.

FIG. 4 illustrates the infrared transmission spectra of a light weightpaper sheet, containing moisture, at two different temperatures withindication of appropriate detected measure, reference, temperaturecorrection, cellulose and synthetic wavelength bands.

FIG. 5 illustrates the infrared transmission spectra of a medium weightpaper sheet, containing moisture, at two different temperatures withindication of appropriate detected measure, reference, temperaturecorrection and cellulose wavelength bands.

FIG. 6 illustrates the infrared transmission spectra of a heavy weightpaper sheet, containing moisture, at two different temperatures withindication of appropriate detected measure, reference, temperaturecorrection, cellulose and second reference wavelength bands.

FIG. 7 illustrates the geometry and nomenclature used for the analysisof the light distribution on two parallel planes.

FIG. 8 illustrates a graph of the absorption and scattering powers of apaper sheet plotted as a function of the reciprocal of the intensity ofradiation individually detected at a transmission and a reflectiondetector which are offset to the same degree from an infrared radiationsource.

FIG. 9 illustrates an alternative embodiment for the source and detectorapertures of the reflective paper guides and a possible variation forthe surface of the paper guides.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description covers the best mode of carrying out theinvention. For the sake of simplicity the invention will be describedprimarily as measuring the amount of moisture in a paper sheet by use ofinfrared radiation. However, this is only to illustrate the principlesof the invention and should not be taken in a limiting sense. Theprinciples of the invention may be also used to measure other selectcomponents or physical properties (e.g., basis weight) of a sheet ofother types of materials (e.g., plastic film) with other forms ofelectromagnetic radiation (e.g., ultraviolet and visible light).Therefore, the scope of the invention is best determined by reference tothe appended claims. In the accompanying drawings like numeralsdesignate like parts.

FIG. 1 illustrates a scanner 10 which includes a framework 12 of a pairof spaced upper and lower parallel beams 14 and 16 extending in thecross-direction across the sheet of material or paper 18. Paper 18travels through the scanner 10 in the direction shown by arrow 20. Lowerand upper gauging heads 22 and 24 are provided on the framework 12 andtravel longitudinally of framework 12 and in the cross-direction ofpaper 18.

FIG. 2 illustrates an infrared moisture sensor 32. It includes a lowerhead 22 with a radiation source for directing infrared radiation 42through a source aperture 34 to paper 18. Favorable results wereachieved by using a radiation source including an incandescent lamp 38and an elliptical reflector 40 with a source aperture 34 of about 1/2inch in diameter. It is preferred, but not necessary to the invention,that the amount of radiation emitted from lamp 38 and falling on paper18 be modulated at a known frequency.

This modulation may be accomplished by any one of several devices. Forexample, the tines 44 of a tuning fork 46 may be disposed in the path ofthe radiation 42. The vibrating tines 44 modulate the radiation 42 asthe tines 44 move alternatively in and out of the path of radiation 42.Alternatively, an opaque disk (not shown) having a plurality of evenlyspaced radial slots may be rotated in the path of the radiation so thatthe radiation is alternately transmitted through the slots and blockedby the opaque portions of the disk. With either device, radiation 42 ismodulated at a known frequency The reason for modulating the radiationis explained below.

The lower and upper heads 22 and 24 include opposing surfaces whichfunction as paper guides 28 and 30. Each of the guides 28 and 30includes a reflective coating 48 (FIGS. 3a and 3b) for directingradiation from source aperture 34 to detector apertures 36 and 37. FIG.3a illustrates that guide 28 includes a reflective coating 48, a sourceaperture 34 and a detector aperture 37. FIG. 3b illustrates that guide30 has a similar reflective coating 48 and a detector aperture 36.Reflective coating 48 is preferably a non-specular or diffuse reflectivesurface. For example, coating 48 may consist of a layer of translucentquartz or glass ceramic backed by a reflective material. To provide aneasily cleaned surface the surfaces of guides 28 and 30 may be anodizedaluminum. In one preferred embodiment, the guides 28 and 30 may consistof two diffuse reflective parallel plates about 0.4 inches apart.

In the embodiment of FIG. 2, radiation 42 is reflected back and forthbetween lower and upper guides 28, 30, before entering detectorapertures 36 and 37. This ensures that radiation 42 makes multipleinteractions with paper 18, that is, passes through the paper 18 anumber of times. This provides certain advantages when measuring themoisture content of very light grades of paper, such as tissue, and veryheavy paper grades. This technique and the advantages of such multipleinteractions with the paper sheet are more fully discussed in U.S. Pat.No. 3,793,524 to Howarth, assigned to Measurex Corporation, which isincorporated herein by reference.

Radiation 42 from source aperture 34 reaches detector apertures 36 and37 by a somewhat complex set of paths, partially illustrated by thedashed lines. The radiation 42 initially impinges on paper 18 with partof the radiation 42 passing through and part being reflected by thepaper 18. Guides 28 and 30 reflect this radiation 42 back to the paper18 where it undergoes the same process of partial transmission andreflection. In addition, the paper 18 itself, being translucent, acts todiffuse the radiation 42 to increase the number of paths.

The mean number of times the radiation 42 passes through the paper 18 onits path from the source aperture 34 to the detector apertures 36 and 37can be easily controlled by adjusting the geometry of sensor 32. In thismanner, paper 18 can be made to appear thicker than its actualthickness.

The radiation 42 enters upper head 24 through detector aperture 36. Theupper head 24 includes a light pipe 50 which guides the radiation 42 toa lens 52 which collimates the radiation. The first beam splitter 54splits the radiation 42 into three separate beams 56, 58 and 60. Bandpass filters 62 and 64 are positioned in the respective paths of beams56 and 60. Lenses 66 and 68 focus the radiation on a temperaturedetector 70 and a synthetic detector 72. Detectors 70 and 72 may be ofthe lead sulfide type. Each filter 62 and 64 is designed to passradiation in a select spectral band. Radiation not within the pass bandof filters 62 and 64 is reflected by these filters to beam splitter 54and does not reach temperature detector 70 or synthetic detector 72.

The portion of radiation 42 transmitted through the first beam splitter54, that is, beam 58, impinges on a second beam splitter 74. The secondbeam splitter 74 splits beam 58 into three beams 76, 84 and 92. Bandpass filters 78, 86 and 94 are positioned in the respective paths ofbeam 76, 84 and 92. Lenses 80, 88 and 96 focus the radiation on acellulose detector 82, a measure detector 90 and a reference detector98. Detectors 82, 90 and 98 also may be of the lead sulfide type. Eachfilter 78, 86 and 94 is selected so that it passes radiation in aseparate band of the spectrum. Thus, a radiation 42 enters the upperhead 24 through detector aperture 36, but the optics in the upper head24 split the radiation 42 into five beams 56, 60, 76, 84 and 92 each ofwhich is detected by an associated infrared detector 70, 72, 82, 90 and98.

In a similar manner, the lower head 22 includes a light pipe 51 whichguides radiation 42 to a collimating lens 53 between the light pipe 51and a first beam splitter 55. The first beam splitter 55 splitsradiation 42 into three separate beams 57, 59 and 61. Band pass filters63 and 65 are positioned in the respective paths of beams 57 and 61.Lenses 67 and 69 focus the radiation on a temperature detector 71 andsynthetic detector 73. Detectors 71 and 73 may be of the lead sulfidetype. Each filter 63 and 65 is selected so that it passes radiation in aseparate band of the spectrum. Radiation not within the pass band offilters 63 and 65 is reflected by these filters to the first beamsplitter 55 and does not reach temperature detector 71 or syntheticdetector 73.

The portion of radiation 42 transmitting through first beam splitter 55,that is, beam 59, impinges on a second beam splitter 75. The second beamsplitter 75 splits the beam 59 into three separate beams 77, 85 and 93.Band pass filters 79, 87 and 95 are positioned in the respective pathsof beam 77, 85 and 93. Lenses 81, 89 and 97 focus the radiation on acellulose detector 83, a measure detector 91 and a reference detector99. Detectors 83, 91 and 99 may be of the lead sulfide type. Each filter79, 87 and 95 is selected so that it passes radiation in a separate bandof the spectrum. Thus, radiation 42 enters the lower head 22 throughdetector aperture 37, but the optics in the lower head 22 split up theradiation 42 into five beams 57, 61, 77, 85 and 93, each of which isdetected by an associated detector 71, 73, 83, 91 and 99.

Bandpass filters in the upper head 24 are made to be substantiallysimilar to the filters in the lower head 22 with the same function.Thus, filter 62 is substantially similar to filter 63; filter 64 issimilar to filter 65; filter 78 is similar to filter 79; filter 86 issimilar to filter 87; and filter 94 is similar to filter 95.

FIG. 9 illustrates an alternative arrangement for source aperture 34 anddetector apertures 36 and 37. The source aperture 34 and detectoraperture 37 have the same locations as that shown in FIG. 2, but thedetector aperture 36 would move to the right. Nonetheless, detectorapertures 36 and 37 would still have substantially the same offset Dfrom the source aperture 34. As shown in FIG. 9, the guides 28 and 30would include a quartz layer 120, 122 and 124 with a reflective backing(not shown) and a radiation absorbing medium 118.

The advantage of this arrangement is it reduces the dependence on thetransmitted portion of the radiation reaching detector aperture 37 andthereby enhances its dependence on scattering. The ultimate advantagewould be in the reduction of number of grade groups required tocalibrate the sensor 32.

FIG. 4 illustrates the infrared transmission spectrum 101 of a lightweight paper sheet, e.g., 70 grams/meter² (g/m²), containing moisture,at a temperature of approximately 22° C. and of 60° C. The cross-hatchedareas denoted MES, REF, CORR, CEL and SYN indicate the wavelength bandsdetected by measure detector 90, reference detector 98, temperaturecorrection detector 70, cellulose detector 82 and synthetic detector 72in upper head 24 (FIG. 2). Similarly, the cross-hatched areas MES, REF,CORR, CEL and SYN indicate the wavelength bands detected by measuredetector 91, reference detector 99, temperature correction detector 71,cellulose detector 83 and synthetic detector 73 in the lower head 22.

To obtain the data for the infrared transmission spectrum 101, twoplates of glass are placed on the opposing sides of a paper sheet andact to prevent loss of moisture during heating between the desiredtemperatures. The plates of glass serve not only to maintain constantmoisture values during the measurements, but also to provide a thermalmass which maintains the paper sheet temperature to make possiblemeasurements at temperatures above the ambient or at a lowertemperature. To accomplish this, the glass-enclosed paper sheet is firstheated, then measurements of the infrared transmission through the glassenclosing the paper sheet are made at the higher and lower temperatures.

The infrared absorption spectrum of water and paper is peculiar in thatthe absorption characteristics of the entire spectrum shift to shorterwavelengths as the paper sheet temperature is increased and to longerwavelengths as the paper sheet temperature decreases. As illustrated inFIG. 4, The infrared spectra of the light weight paper sheet at thehigher temperature is shown by the dashed line 102. The infrared spectraat the lower temperature is shown by the solid line 104. Of course, theinfrared spectrum 102 of the paper sheet at the higher temperature hasapproximately the same absorption characteristic as the lowertemperature paper sheet, but at shorter wavelengths.

The infrared spectrum is affected by both the absorption by water and bypaper fibers. In the band around 1.93 microns wavelength, water is muchmore efficient at absorbing infrared radiation than paper fibers. Thus,in this band of the spectrum the absorption is most strongly affected bythe water content of the paper 18.

As shown in FIG. 4, for a light weight paper sheet (e.g., 70 g/m²), aninfrared band pass filter 86 (FIG. 2) associated with a measure detector90 may have its pass band 112 approximately centered around the waterabsorption peak 106, for example, at approximately 1.93 microns. Forthis purpose, we may use a band pass filter 86 (FIG. 2) with a rangefrom 1.92 to 1.95 microns, the lower and upper wavelengths at which thetransmission reaches half that which is achieved at the transmissionpeak.

In this way, as the sheet temperature increases, the intensity ofdetected infrared radiation in the MES band increases at the longwavelength side of the band, while an approximately equal decrease indetected infrared radiation occurs at the opposite short wavelength sideof the band. With this technique, the total amount of infrared radiationreaching the measure detector 90 is strongly sensitive to the moisturecontent and substantially insensitive to sheet temperature. Thus, thesignal from measure detector 90 (the "MES" signal) provides a roughmeasurement of the sheet moisture content which is substantiallyinsensitive to temperature change.

As mentioned earlier, the basis weight of the paper 18 also affects theinfrared transmission spectrum. To provide a signal which is sensitiveto the basis weight, as shown in FIG. 2, a band pass filter 94 ispositioned before a reference detector 98. The filter 94 has its passband 110 (FIG. 4) defining a REF band which is less sensitive to waterand substantially insensitive to the sheet temperature. For example, afilter with a band pass range from 1.82 to 1.86 microns (normalincidence) has both of these characteristics. Because the pass band 110of the filter is less than 1.9 microns, it is sensitive to the basisweight of paper 18. Accordingly, as the basis weight increases, theamount of infrared radiation passing through the sheet decreases. Thus,the signal from detector 98 (the "REF" signal) provides a roughmeasurement of the basis weight of the paper.

Because the MES and REF signals may be sensitive to the sheettemperature, the invention provides a temperature correction detector 70(FIG. 2) for temperature correction with an associated band pass filter62. The signal from this temperature correction detector 70 (the "CORR"signal) may be used to correct the measurement of the sensor 32 for theeffects of varying sheet temperature as described below. The pass band108 chosen for this filter 62 passes radiation in a band of thetransmission spectrum 101 so that the amplitude of the signal from thetemperature correction detector 70 is sensitive to changes in the sheettemperature. The changes in the CORR signal from the temperaturecorrection detector 70 are used to compensate for temperature inducedchanges in the MES and/or REF signals.

A preferred position for the detected CORR band or pass band 108 of thisfilter is shown in FIG. 4. For example, a band pass filter of 1.68 to1.72 microns passes a band of the infrared spectrum where favorabletemperature correction has been achieved for light weight paper.

The invention may also provide a cellulose detector 82 (FIG. 2) with anassociated band pass filter 78. The signal from this cellulose detector82 (the "CEL" signal) may be used to correct the moisture measurementfor varying cellulose content or to provide a measurement of themoisture content as a percentage of the total sheet weight. The passband 114 chosen for this filter 78 passes radiation in a band of thetransmission spectrum 101 that is sensitive to the cellulose content ofthe paper 18.

A preferred position for the detected CEL band or pass band 114 of thisfilter is shown in FIG. 4. For example, a band pass filter of 2.06 to2.10 microns passes a band of the infrared spectrum where favorabletemperature correction has been achieved for light weight paper.

The invention may also provide a synthetic detector 72 (FIG. 2) with anassociated band pass filter 64. The signal from this synthetic detector72 (the "SYN" signal) may be used to correct the moisture measurementfor varying synthetic content. Although relatively less common in paperproducts, synthetic fibers (e.g., polyester fibers and polyethylenefiber) may be included in light weight paper products (e.g., tea bags)to strengthen the paper to avoid bursting when wet. The pass band 116chosen for this filter 72 passes radiation in a band of the transmissionspectrum 101 that is sensitive to the synthetic fiber content of thepaper 18.

A preferred position for the detected SYN band or pass band 108 of thisfilter is shown in FIG. 4. For example, a band pass filter of 2.33 to2.37 microns passes a band of the infrared spectrum where favorablesynthetic detection has been achieved for light weight paper.

It is preferred that an identical set of band pass filters be arrangedin like manner in the lower head 22.

FIG. 5 illustrates the infrared transmission spectrum 101 for a mediumweight paper sheet (e.g., a 205 g/m² liner board), containing moistureat a temperature of approximately 22° C. and of 60° C. The infraredspectrum of the sheet at the higher temperature is shown by the dashedline 102. The infrared spectrum of the sheet at the lower temperature isshown by the solid line 104. To obtain the infrared spectrum, the papersheet was sealed between two plates of glass to prevent loss of moistureduring heating. Then measurements of the infrared penetration throughthe glass enclosed sheets were made at the higher and lower temperature.The cross-hatched areas denoted MES, REF, CORR and CEL again illustratethe separate wavelength bands to be passed through the respectivefilters and detected.

As mentioned earlier, a heavier weight paper product typically containsmore moisture than a light weight paper. As the amount of moistureincreases, the water absorption peak increases in magnitude as well asbroadens in the wavelength direction. As illustrated by FIG. 5, both ofthese effects tend to reduce the amount of radiation transmitted throughthe sheet. In fact, at and around the water absorption peak, the strongwater and cellulose absorptions of a heavier grades of paper mayeffectively absorb much of the infrared radiation directed at the sheetfrom the infrared source. Thus, the narrow band pass filter as used forthe light weight paper discussed earlier may be entirely inadequate interms of passing the required amount of radiation to the detector.

Rather than selecting such a narrow band pass filter to pass infraredradiation in a band adjacent to the water absorption peak, the inventionovercomes the problem of relatively low transmission by providing arelatively broad band pass filter around the water absorption peak. Thisalso minimizes temperature sensitivity by ensuring that the integratedareas beneath the infrared transmission spectrum for a wide range oftemperatures remain roughly equal and that the water absorption peakremains within the filter envelope.

For this medium weight paper sheet, an infrared band pass filter 86(FIG. 2) associated with the measure detector 90 may have its pass band112 around the water absorption peak 106. For this purpose, we may use aband pass filter 86 (FIG. 2) with a range from 1.88 to 2.04 microns (atnormal incidence).

In this way, as the sheet temperature increases, the absorption peakremains within the pass band 112, and the intensity of detected infraredradiation in the MES band increases at the long wavelength side of theband, while an approximately equal decrease in detected infraredradiation occurs at the opposite short wavelength side of the band. Withthis technique, the total amount of infrared radiation reaching themeasure detector 90 is strongly sensitive to the moisture content andsubstantially insensitive to sheet temperature. This is because totalamount of infrared radiation reaching the measure detector 90 isproportional to the integrated area underneath the transmission curve.Thus, the signal from measure detector 90 (the "MES" signal) provides arough measurement of the sheet moisture content which is substantiallytemperature insensitive.

As previously mentioned, the infrared absorption spectrum is alsoaffected by the basis weight of the paper 18. To provide a signaldependent upon the basis weight of the paper 18, a band pass filter 94is positioned before a reference detector 98. As shown by FIG. 5, thisfilter 94 has its pass band 110 at a wavelength band which is lesssensitive to water and substantially insensitive to the sheettemperature. For example, a band pass filter with a range from 1.82 to1.86 microns (normal incidence) has both of these characteristics.Because the pass band 110 of this filter 94 is less than 1.9 microns, itis also sensitive to the basis weight of the paper 18. Accordingly, asthe basis weight increases, the amount of infrared radiation whichpasses through the paper 18 decreases. Thus, the signal from thisreference detector 98 (the "REF" signal) provides a rough measurement ofthe basis weight of the paper.

As in the measurement of the light weight paper, because the MES and REFsignals may still be sensitive to the sheet temperature, the inventionprovides a temperature correction detector 70 (FIG. 2) for temperaturecorrection with an associated infrared band pass filter 62. The signalfrom this temperature correction detector 70 (the "CORR" signal) may beused to correct the moisture measurement of the sensor 32 for theeffects of varying sheet temperature. The pass band 108 chosen for thisfilter 62 passes radiation in a band of the transmission spectrum 101 sothat the amplitude of the signal from the temperature correctiondetector 70 is sensitive to changes in the sheet temperature. Thechanges in the CORR signal from the temperature correction detector areused to compensate for the temperature induced changes in the MESsignal.

A preferred position for the detected CORR band or pass band 108 of thisfilter is shown in FIG. 5. For example, a band pass filter of 1.68 to1.72 microns passes a band of the infrared spectrum where favorabletemperature correction has been achieved for medium weight paper.

The invention may also provide a cellulose detector 82 (FIG. 2) with anassociated band pass filter 78. The signal from this cellulose detector82 (the "CEL" signal) may be used to correct the moisture measurementfor varying cellulose content. The pass band 114 chosen for this filter78 passes radiation in a band of the transmission spectrum 101 that issensitive to the cellulose content of the paper 18.

A preferred position for the detected CEL band or pass band 114 of thisfilter is shown in FIG. 5. For example, a band pass filter of 2.06 to2.10 microns passes a band of the infrared spectrum where favorabletemperature correction has been achieved for medium weight paper.

Again it is preferred that an identical set of band pass filters bearranged in like manner in the lower head 22.

FIG. 6 illustrates the infrared transmission spectrum 101 for a heavyweight paper sheet (e.g., a 345 g/m² liner board), containing moisture,at a temperature of approximately 22° C. and of 60° C. As in FIGS. 4-5,the infrared spectrum of the sheet at the higher temperature is shown bythe dashed line 102. The infrared spectrum of the sheet at the lowertemperature is again shown by the solid line 104. To obtain the infraredspectrum, the paper sheet was again sealed between two plates of glassto prevent loss of moisture during heating then measurements of theinfrared penetration through the glass-enclosed sheets are made at thehigher temperature and at the ambient or room temperature. Thecross-hatched areas designated MES, REF, CORR, CEL and REF2 illustratethe separate wavelength bands to be passed through the respectivefilters and detected.

As mentioned earlier, a heavier grade of paper typically contains moremoisture than a lighter weight paper. As the amount of moistureincreases even more than that contained in a medium weight paper sheet,the water absorption peak increases in magnitude as well as broadens inthe wavelength direction even more. As illustrated by FIG. 6, both ofthese effects tend to further reduce the amount of radiation transmittedthrough the sheet. In fact, around the water absorption peak, the strongwater and cellulose absorptions of the heavier grades of paper absorbmuch of the infrared radiation directed at the sheet from the infraredsource. Thus, a narrow band pass filter as specified for the lightweight paper, or even the broader band pass filter specified for themedium weight paper may be inadequate to pass the required amount ofradiation to the detector.

The invention overcomes the problem of relatively low transmission byproviding an even broader band pass filter around the water absorptionpeak than in the situations illustrated in FIGS. 4 and 5. This alsoreduces temperature sensitivity by ensuring the water absorption peakremains within the filter envelope.

For this heavy weight paper sheet, an infrared band pass filter 86 (FIG.2) associated with the measure detector 90 may have its pass band 112around the water absorption peak 106. For this purpose, we may use aband pass filter 86 (FIG. 2) with a range from 1.88 to 2.04 microns (atnormal incidence).

In this way, as the sheet temperature increases the peak remains withinthe pass band, and the intensity of the infrared radiation increases onthe long wavelength half of the filter 86, while a decrease in theintensity of the infrared radiation occurs at the opposite shortwavelength half of the filter 86. However, this technique does notresult in temperature insensitivity, because the total amount ofinfrared radiation reaching the measure detector 90 may not besubstantially equal for the high and low temperatures. Thus, themeasurement detector 90 is not only strongly dependent upon the moisturecontent of the paper 18, but may also be sensitive to the sheettemperature. Thus, the signal from measure detector 90 (the "MES"signal) provides a moisture measurement which may be temperaturesensitive.

The infrared absorption spectrum is affected by the basis weight of thepaper 18. To provide a signal sensitive to the basis weight of the paper18, a band pass filter 94 is positioned before a reference detector 98.To compensate for any temperature sensitivity of the MES signal, thefilter 94 has its pass band 110 defining a REF band which is lesssensitive to the water in the sheet than the MES band surrounding thewater absorption peak, but is sensitive to the sheet temperature. Forexample, a filter with a band pass range from 1.82 to 1.86 microns(normal incidence) has these characteristics. Because the pass band 110of this filter 94 is less than 1.9 microns, it is also sensitive to thebasis weight of the paper 18. Accordingly, as the basis weightincreases, the amount of infrared radiation which passes through thepaper 18 decreases. Thus, the signal from this detector 98 (the "REF"signal) provides a rough measurement of the basis weight of the paper.

Because the MES and REF signals may be sensitive to the sheettemperature, the invention provides a temperature correction detector 70(FIG. 2) for temperature correction with an associated band pass filter62. The signal from this temperature correction detector 70 (the "CORR"signal) may be used to correct the moisture measurement of the sensor 32for the effects of varying sheet temperature. The pass band 108 chosenfor this filter 62 passes radiation in a band of the transmissionspectrum 101 so that the amplitude of the signal from the temperaturecorrection detector 70 is sensitive to changes in the sheet temperature.The changes in the CORR signal from the temperature correction detectorare used to compensate for the temperature induced changes in the MESsignal.

A preferred position for the detected CORR band or pass band 108 of thisfilter is shown in FIG. 6. For example, a band pass filter of 1.68 to1.72 microns passes a band of the infrared spectrum where favorabletemperature correction has been achieved for medium weight paper.

The invention may also provide a cellulose detector 82 (FIG. 2) with anassociated band pass filter 78. The signal from this cellulose detector82 (the "CEL" signal) may be used to correct the moisture measurementfor varying cellulose content. The pass band 114 chosen for this filter78 passes radiation in a band of the transmission spectrum 101 that issensitive to the cellulose content of the paper 18.

A preferred position for the detected CEL band or pass band 114 of thisfilter is shown in FIG. 6. For example, a band pass filter of 1.47 to1.53 microns passes a band of the infrared spectrum where favorabletemperature correction has been achieved for heavy weight paper.

Finally, the invention may also provide a second reference detector withan associated band pass filter. Although the second reference detectoris not shown in FIG. 2, the second reference detector and associatedfilter could be disposed at the same physical location as the syntheticdetector 72 and filter 78. The REF2 band or pass band 126 chosen forthis filter passes radiation in a band of the transmission spectrum 101that is less sensitive to the cellulose than the cellulose detector 82.Thus, the cellulose reference detector serves as a reference.

A preferred position for the REF2 band or pass band 126 of this filteris shown in FIG. 6. For example, a band pass filter of 1.30 to 1.34microns passes a band of the infrared spectrum where favorable resultshave been achieved for heavy weight paper.

It is preferred that an identical set of band pass filters be arrangedin like manner in the lower head 22.

Table 1 gives the appropriate band pass filters for basis weights of upto 550 g/m². Band pass ranges are expressed as the lower and upperwavelengths at which the transmission reaches half that which isachieved at the transmission peak.

                                      TABLE 1                                     __________________________________________________________________________    Maximum                               SYN/REF2                                Basis MES Filter                                                                            REF Filter                                                                            CORR Filter                                                                           CEL Filter                                                                            Filter                                  Weight                                                                              microns microns microns microns microns                                 g/m.sup.2                                                                           lower                                                                             upper                                                                             lower                                                                             upper                                                                             lower                                                                             upper                                                                             lower                                                                             upper                                                                             lower                                                                             upper                               __________________________________________________________________________     70   1.92                                                                              1.95                                                                              1.82                                                                              1.86                                                                              1.68                                                                              1.72                                                                              2.06                                                                              2.10                                                                              2.33                                                                              2.37                                150   1.90                                                                              2.01                                                                              1.78                                                                              1.82                                                                              1.68                                                                              1.72                                                                              2.06                                                                              2.10                                                                              --  --                                  250   1.88                                                                              2.04                                                                              1.82                                                                              1.86                                                                              1.68                                                                              1.72                                                                              2.06                                                                              2.10                                                                              --  --                                  325   1.87                                                                              2.05                                                                              1.82                                                                              1.86                                                                              1.68                                                                              1.72                                                                              1.47                                                                              1.53                                                                              1.30                                                                              1.34                                550   1.84                                                                              2.03                                                                              1.82                                                                              1.86                                                                              1.68                                                                              1.72                                                                              1.47                                                                              1.53                                                                              1.30                                                                              1.34                                __________________________________________________________________________

The "Maximum Basis Weight" column specifies the maximum basis weight ing/m² of the paper product for which the filter set should be used. Themaximum water weight is approximately no more than 10% for each grade ofpaper product. Thus, for a particular context, the customer specifiesthe maximum basis weight and the maximum percent moisture for the paperbeing manufactured and the appropriate filter sets are then selectedfrom Table 1 which satisfy both of these conditions.

As shown in FIG. 2, the infrared radiation from lamp 38 is modulated bythe tines 44 of the vibrating tuning fork 46. For the sake of simplicitythe modulating of the radiation 42 is explained for the upper head 24alone. However, the same arrangement is also preferred for the lowerhead 22. The output of each detector 70, 72, 82, 90 and 98 issinusoidally modulated at the same frequency and phase as the detectedinfrared beams 56, 60, 76, 84 and 92. However, infrared energy from thepaper 18 itself and from other external sources (not shown) will alsoreach the detectors. Thus, each detector signal also includes a DCcomponent.

The output of each of the five detectors 70, 72, 82, 90 and 98 istransmitted to the signal processing circuitry 45. The circuitry 45 isdesigned to filter out the DC component of the detector signals. Thefiltered detector signals are then passed on to a phase synchronousdemodulation circuit included within the signal processing circuitry 45.The purpose of the phase synchronous demodulator is to filter outchanges in the signals from the detectors 70, 72, 82, 90 and 98 whichare not caused by the varying infrared absorption of the paper 18. Forexample, 60 Hz line noise in the detector signals is filtered out by thedemodulator circuit, as explained below.

A sine wave oscillator 43 drives the tines 44 of tuning fork 46 at itsresonant frequency. The output of this oscillator 43 is converted to asquare wave with the same frequency and phase as the sine waves drivingthe tuning fork 46. This square wave output 41 is fed to a phasesynchronous demodulator portion of the signal processing circuitry 45,along with the filtered signals from each of the five detectors 70, 72,82, 90 and 98. Of course, the filtered signals are modulated at the samefrequency and phase as the output of oscillator 43. By demodulating theoutputs from each of the detectors 70, 72, 82, 90 and 98 with a squarewave having the same frequency and phase as the output of the oscillator43 and averaging the demodulated outputs over a number of cycles, thesensor 32 filters out detector signals changes from changes in theintensity of external infrared sources or extraneous signals such as the60 Hz line voltage. This filtering technique using a phase synchronizeddemodulation circuit is known. This reduces erroneous moisturemeasurements. The output signal of each detector indicates the intensityof radiation 42 passing through the associated band pass filter.

The invention provides a graph as shown in FIG. 8 from a mathematicalanalysis for determining the scattering and absorption power of diffusemedia such as paper. The scattering and absorption powers are determinedto be a function of the intensity of the detected radiation at both thelower and upper heads of the moisture sensor. The following analysisdepends in part on the Kubelka-Munk theory. This theory describes thebehavior of light interacting with diffuse media such as paper andprovides a mathematical analysis for determining the amount of lighttransmitted through and reflected from the paper. W. Wendlandt and H.Hecht, Reflectance Spectroscopy, Chapter III (1966) provides adescription of the Kubelka-Munk theory and is incorporated herein byreference.

As shown in FIG. 7, the technique is best illustrated by an examplewhich involves two parallel planes, S and P. It will be assumed that theplanes act as Lambertian surfaces, that is, they are perfect diffusersof radiation and follow Lambert's law:

    I(Φ)=I.sub.o cos(Φ)

where:

Φ=the angle from the normal to the surface and the direction of lightleaving the surface

I(Φ)=the intensity of light per unit solid angle leaving the surface inthe direction Φ

I_(o) =the intensity of light per unit solid angle leaving the surfacein the normal direction.

It also will be assumed that a radiation source is in plane S and isdistributed symmetrically about an axis normal to the surfaces of planesand centered in the middle of the illuminated area of plane. Finally, itwill be assumed that the sheet being measured acts as a diffusingmaterial such as paper and has the reflection and transmissionproperties described by the theory of Kubelka and Munk.

The first step in the analysis is to determine the distribution of lightincident on the surface of the parallel sheet facing the source fromlight coming from the source plane. The process used is that for everypoint on the parallel sheet, the total intensity of light reaching itfrom every point on the source plane is calculated.

FIG. 7 shows the two parallel planes, S and P. Consider an element ofarea da_(s) at a point on the S plane located at radial distance r_(s)and at angle Θ_(s) from the Y axis. The intensity of light per unit areaper unit solid angle leaving normal to that surface at that point isgiven to be i_(s) (r_(s)) (watts/cm² *sr). The total light intensityreaching an element of area da_(p) on plane P at r_(p) and Θ_(p) fromda_(s) is given by:

    d.sup.2 F(r.sub.p,r.sub.s,Θ.sub.s,Θ.sub.p,Φ)=i.sub.s (r.sub.s) cos(Φ) da.sub.s (da.sub.p cos(Φ)/r.sup.2) (watts)

where r is the distance between da_(s) and da_(p). The first cos(Φ) termon the right is from applying Lambert's law. The term in parentheses onthe right is the solid angle subtended by da_(p). The angle Φ is betweenthe normal to the planes and a line drawn between da_(s) and da_(p). Theintensity per unit area incident on da_(p) from da_(s) is:

    df(r.sub.p,r.sub.s,Θ.sub.s,Θ.sub.p,Φ)=i.sub.s (r.sub.s) da.sub.s (cos.sup.2 (Φ)/r.sup.2) (watts/cm.sup.2)

The value of cos(Φ) can be written in terms of the separation betweenplanes (1) and r:

    cos(Φ)=l/r

The intensity per unit area becomes:

    df(r.sub.p,r.sub.s,Θ.sub.s,Θ.sub.p,Φ)=i.sub.s (r.sub.s) da.sub.s (l.sup.2 /r.sup.4) (watts/cm.sup.2)

The value of r can be written in terms of the position variables r_(p),r_(s), Θ_(s) and Θ_(p) :

    r=sqrt(l.sup.2 +r.sub.s.sup.2 +r.sub.p.sup.2 -2r.sub.s r.sub.p cos(Θ.sub.s -Θ.sub.p))

The incremental area da_(s) can also be written in terms of the positionvariables r_(s) and Θ_(s).

    da.sub.s =r.sub.s dr.sub.s dΘ.sub.s

Combining we have the intensity per unit area in terms of positionvariables only: ##EQU1## To obtain the total intensity per unit area onelemental area da_(p) on plane P from the entire surface of plane S, theequation above must be integrated over the entire surface from r_(s) =0to r_(s) =r_(smax), the maximum radius of plane S and Θ_(s) =0 to 2π:##EQU2##

This integral is best solved by a numerical method. One suitable methodis the trapezoidal method. To obtain the distribution of incidentintensity on plane P, the integral must be solved at all positions ofr_(p) and Θ_(p).

If plane P represents a sheet of diffusing material between two planes,S and T (not shown), the distribution of incident light on plane P canbe used to calculate the intensity of the reflected and transmittedlight. If the sheet is paper, the theory of Kubelka-Munk provides anaccurate way to calculate the reflected and transmitted light when theabsorption and scattering coefficients are known. The transmissioncalculated from Kubelka-Munk theory multiplied by the incident lightdistribution on plane P is now a source distribution to plane T.Similarly, the reflection calculated from Kubelka-Munk theory multipliedby the incident light distribution on plane P is now a sourcedistribution back to the plane S.

The same method is used again to determine the distribution of incidentlight on plane T. The incident light distribution on plane T isreflected back, becoming a source distribution of light which impingeson the back side of plane P. This light is reflected and transmitted byplane P as described above. The light thus transmitted adds to the lightthat was incident from plane S and is reflected back from plane P. Thisforms a new distribution of light that is now incident on the plane S.

The new distribution of light on plane S is the incident light fromplane P reflected back plus the original illuminated area. With this newsource distribution the calculations are repeated to find newdistributions on all planes. After several iterations there will be nochange, that is the results will have converged to a final value andadditional iterations of the calculations will not produce differentresults.

Once the light distribution is known for planes S and T, the calculatedlight intensity can be determined for a detector placed anywhere onplane S or on plane T. Two positions have been analyzed in detail, butthe same approach would work for any position. The positions analyzedare where: (1) the detector is on plane T and offset from theilluminated area; and (2) the detector is on plane S and offset the samedistance from the source of light or the illuminated area. The former isreferred to as a transmission detector and the latter as a reflectiondetector.

Up to this point we have shown that if the absorption and scatteringcoefficients of the sheet are known the intensity distribution of lighton all planes can be calculated. In actual operation, a sensor would beset up to measure the intensity at two or more detector locations andthen absorption and scattering coefficient would be determined. Toimplement this technique, the intensity distributions are calculated forall values of absorption and scattering coefficients likely to beencountered.

FIG. 8 illustrates a graph of the absorption and scattering powers of apaper sheet plotted as a function of the intensity of radiationindividually detected at a transmission and a reflection detector whichare offset to the same degree from an infrared radiation source. Theordinate of the graph is determined by calculating an intensity ratiofrom the transmission detector located on plane T. The abscissa isdetermined by calculating an intensity ratio from the reflectiondetector located on plane S. The intensity ratio for both detectors isdefined as the intensity with no sheet of material between plane S and Tto the intensity when the sheet of material is interposed therebetween.Contour lines connect equal absorption powers and equal scatteringpowers associated with those ratios.

To use a graph as illustrated in FIG. 8 the sheet to be measured isplaced between the planes S and T. The intensity of a select signal,such as the MES signal, is measured by the detectors at the twolocations. Next, the intensity is determined when there is no sheetinterposed between planes S and T. The intensity ratio, that is, theintensity with no sheet in interposed divided by intensity with thesheet interposed is then calculated. The intensity ratios are then usedto find the absorption and scattering powers of the sheet. Of course, byinterpolating between the contour lines of equal absorption powers andequal scattering powers the actual value of absorption and scatteringcoefficients can be determined.

Two approaches can be used to correct temperature error in theabsorption measurements. Under the first approach, we can assume thatthe temperature dependent part of the absorption coefficient can beseparated:

    k.sub.i (T)=k.sub.io f.sub.i (T)                           (1)

where T=temperature;

f_(i) (T) is a function that depends on temperature alone; and k_(io)=absorption coefficient of signal i at the calibration temperature.

We can also assume that the temperature function for any signal is alinear function of the temperature function of the correction signal:

    f.sub.i (T)=αf.sub.corr (T)+β                   (2)

By combining (1) and (2) we have: ##EQU3## Then for each signal thecorrected absorption coefficient is: ##EQU4## and α_(i) and β_(i) wouldbe determined experimentally.

A second approach to removing temperature error is to correct the ratiosbefore determining the absorption coefficients. For each wavelength wehave signal taken with no sheet at Standardize, REFS, and a signalon-sheet, REF. We then calculate a ratio for that wavelength: ##EQU5##Calculate a Temperature Correction Ratio: ##EQU6## Coefficients C1, C2and C3 are chosen so that RT is approximately zero at calibrationtemperature. Use this value to correct the signal for each wavelength:##EQU7##

Once the absorption coefficients are known for the moisture andreference wavelengths the moisture calculation is straight forward. Forthe simple case of plain paper with no fillers or broad band absorbers:

    k=k.sub.water *%MOI/100+k.sub.fiber *(1-%MOI/100)

where

k=absorption coefficient of paper at 1.94 microns determined by theprocess described above

k_(water) =absorption coefficient of water (a known constant)

k_(fiber) =absorption coefficient of fiber (a known constant)

%MOI=percent moisture in paper

solving for %MOI

    %MOI=(k-k.sub.fiber)*100/(k.sub.water -k.sub.fiber)=ak+b

where a is a constant equal to 100/(k_(water) -k_(fiber)) and is in aconstant equal to k_(fiber) *a

Thus, in principle a single wavelength is sufficient to determinemoisture in the simplest case.

In the more realistic case where broad band absorbers are present thentwo wavelengths are required: ##EQU8## where

kmes=absorption coefficient of paper at 1.94 micron

kmes_(water) =absorption coefficient of water at 1.94 micron

kmes_(fiber) =absorption coefficient of fiber at 1.94 micron

kmes_(bb) =absorption coefficient of broad band absorber at 1.94 micron

kref=absorption coefficient of paper at 1.8 micron

kref_(water) =absorption coefficient of water at 1.8 micron

kref_(fiber) =absorption coefficient of fiber at 1.8 micron

kref_(bb) =absorption coefficient of broad band absorber at 1.8 micron

bbwt=broad band absorber basis weight

Since the absorption coefficients of the broad band absorber at the mesand ref wavelengths are the same, the difference between kmes and krefgive the moisture without the effect of broadband absorption: ##EQU9##where

dk=kmes-kref;

a=100/(dk_(water) -dk_(fiber)); and

b=-dk_(fiber) -a

Sometimes a better approximation is obtained by using a polynomial of dkwhere

    %MOI=adk+b(dk).sup.2 +C

Based on these measurements, a sheet moisture correction can beaccomplished manually. However, many modern paper mills are highlyautomated. In these paper mills, the signals produced by the sensor 32are preferably fed to a computer 47 which computes the sheet moistureprofile using the signals from the detectors and then, based on thiscomputation, selectively activates one or more known devices 49 foraltering the moisture content of certain portions of the paper 18. Manysuch devices 49 for altering the sheet moisture profile exist, includingsuch devices as selectively controllable water showers for increasingthe moisture content of select cross-directional sections of paper 18and/or infrared heaters for selectively drying such sections of paper18.

We claim:
 1. A sensor for measuring one or more select components of asheet, comprising:a radiation source for emitting radiation toward thesheet; a plurality of detecting means for detecting radiation afterinteraction with the sheet; and means for directing the radiation sothat the radiation makes multiple interactions with the sheet in movingfrom the source to the detecting means, wherein the directing meansincludes a first reflector and a second reflector defining a sheet spacefor the sheet to occupy, wherein the first reflector includes a sourceaperture for emitting radiation and a first detector aperture, offsetfrom the source aperture, for directing radiation to at least onedetecting means and wherein the second reflector includes a seconddetector aperture, offset substantially the same distance as the firstdetector aperture from the source aperture, for directing radiation toat least one detecting means.
 2. The sensor of claim 1, wherein thefirst and second reflectors include diffuse surfaces facing andsubstantially parallel to the sheet.
 3. The sensor of claim 1, furthercomprising means for computing a ratio of the intensity of the detectedradiation when the sheet is absent from the sheet space and theintensity of the detected radiation when the sheet occupies the sheetspace.
 4. The sensor of claim 3, further comprising means for computinga first ratio based on the radiation received at the first detectoraperture and a second ratio based on the radiation received at thesecond detector aperture.
 5. The sensor of claim 4, further comprisingmeans for computing an absorption and scattering power of the sheet fromthe first and second intensity ratios.
 6. The sensor of claim 5, whereinthe detecting means includes a first detector for detecting andgenerating a first signal indicative of the intensity of the radiationin a first band of the spectrum which is sensitive to a first componentof the sheet.
 7. The sensor of claim 6, wherein the detecting meansincludes a second detector for detecting and generating a second signalindicative of the intensity of the radiation in a second band of thespectrum which is less sensitive to the first component than the firstband.
 8. The sensor of claim 7, wherein the detecting means includes athird detector for detecting and generating a third signal indicative ofthe intensity of the radiation in a third band of the spectrum which issensitive to temperature.
 9. The sensor of claim 8, further comprisingmeans for computing a first, second and third absorption powers, of thesheet based on the first, second and third signals.
 10. The sensor ofclaim 9, further comprising means for correcting the first and thesecond absorption powers for the temperature of the sheet based on thethird absorption power.
 11. The sensor of claim 9, wherein the detectingmeans includes a fourth detector for detecting and generating a fourthsignal indicative of the intensity of the radiation at a fourth band ofthe spectrum which is sensitive to a second component of the sheet. 12.The sensor of claim 11, wherein the detecting means includes a fifthdetector for detecting and generating a fifth signal indicative of theintensity of the radiation at a fifth band of the spectrum which issensitive to a third component in the sheet.
 13. The sensor of claim 12,further comprising means for computing a fourth absorption power of thesheet from the fourth signal.
 14. The sensor of claim 13, furthercomprising means for computing a fifth absorption power of the sheetfrom the fifth signal.
 15. The sensor of claim 14, further comprisingmeans for correcting the fourth and fifth absorption powers for thesheet temperature based on the third absorption power.
 16. The sensor ofclaim 12, wherein the sheet is paper, the first component is moisture,the second component is cellulose, the third component is syntheticfiber and the radiation is in the infrared spectrum.
 17. A sensor formeasuring one or more select components of a sheet, comprising:a sourcefor emitting radiation through a source aperture; means for detectingradiation after the radiation has multiple interactions with the sheet,including at least two detector apertures, with substantially the sameoffset from the source aperture, for receiving the radiation; and firstand second reflector means for directing the radiation so that theradiation has multiple interactions with the sheet when moving from thesource to the detecting means.
 18. A method for measuring the amount ofa select component of a sheet comprising the steps of:emitting radiationfrom a source to the sheet; detecting and generating a plurality ofsignals indicative of the intensity of the radiation emerging from thesheet at a plurality of detecting means with substantially equal offsetfrom the radiation source; and directing the radiation so that theradiation makes multiple interactions with the sheet in moving from thesource to the detecting means.
 19. The method of claim 18, furthercomprising the step of:calculating, with a computer, the amount of theselect component using the plurality of signals.
 20. A sensor formeasuring one or more select components of a sheet, comprising:aradiation source for emitting radiation toward the sheet; a plurality ofdetecting means, offset substantially the same from the source, fordetecting radiation after interaction with the sheet, wherein thedetecting means includes a first detector for detecting and generating afirst signal indicative of the intensity of the radiation in a firstband of the spectrum which is sensitive to a first component of thesheet, a second detector for detecting and generating a second signalindicative of the intensity of the radiation in a second band of thespectrum which is less sensitive to the first component than the firstband, a third detector for detecting and generating a third signalindicative of the intensity of the radiation in a third band of thespectrum which is sensitive to temperature, and a fourth detector fordetecting and generating a fourth signal indicative of the intensity ofthe radiation at a fourth band of the spectrum which is sensitive to asecond component of the sheet; means for directing the radiation so thatthe radiation makes multiple interactions with the sheet in moving fromthe source to the detecting means, wherein the directing means includesa first reflector and second reflector defining a sheet space for thesheet to occupy; means for computing a ratio of the intensity of thedetected radiation when the sheet is absent from the sheet space and theintensity of the detected radiation when the sheet occupies the sheetspace; means for computing a first intensity ratio based on theradiation received at a first detector aperture and a second intensityratio based on the radiation received at a second detector aperture; andmeans for computing an absorption and scattering power of the sheet fromthe first and second intensity ratios.
 21. The sensor of claim 20,wherein the detecting means includes a fifth detector for detecting andgenerating a fifth signal indicative of the intensity of the radiationat a fifth band of the spectrum which is sensitive to a third componentin the sheet.
 22. The sensor of claim 21, further comprising means forcomputing a fifth absorption power of the sheet from the fifth signal.23. The sensor of claim 21, wherein the sheet is paper, the firstcomponent is moisture, the second component is cellulose, the thirdcomponent is synthetic fiber and the radiation is in the infraredspectrum.
 24. The sensor of claim 20, further comprising means forcomputing a fourth absorption power of the sheet from the fourth signal.25. A sensor for measuring one or more select components of a sheet,comprising:a radiation source for emitting radiation toward the sheet; aplurality of detecting means, offset substantially the same from thesource, for detecting radiation after interaction with the sheet,wherein the detecting means includes a first detector for detecting andgenerating a first signal indicative of the intensity of the radiationin a first band of the spectrum which is sensitive to a first componentof the sheet, a second detector for detecting and generating a secondsignal indicative of the intensity of the radiation in a second band ofthe spectrum which is less sensitive to the first component than thefirst band, a third detector for detecting and generating a third signalindicative of the intensity of the radiation in a third band of thespectrum which is sensitive to temperature, a fourth detector fordetecting and generating a fourth signal indicative of the intensity ofthe radiation at a fourth band of the spectrum which is sensitive to asecond component of the sheet, and a fifth detector for detecting andgenerating a fifth signal indicative of the intensity of the radiationat a fifth band of the spectrum which is sensitive to a third componentin the sheet; means for directing the radiation so that the radiationmakes multiple interactions with the sheet in moving from the source tothe detecting means, wherein the directing means includes a firstreflector and second reflector defining a sheet space for the sheet tooccupy; means for computing a ratio of the intensity of the detectedradiation when the sheet is absent from the sheet space and theintensity of the detected radiation when the sheet occupies the sheetspace; means for computing a first ratio based on the radiation receivedat a first detector aperture and a second intensity ratio based on theradiation received at a second detector aperture; means for computing anabsorption and scattering power of the sheet from the first and secondintensity ratios; means for computing first, second, third, fourth andfifth absorption powers based on the first, second third, fourth andfifth signals; and means for correcting the first, second, fourth andfifth absorption powers for the sheet temperature based on the thirdabsorption power.